The present invention relates to motor bridge driver circuits for multiphase motor control systems. It is particularly suitable for permanent magnet AC motors, but is also applicable to other types of electric motor such as DC brushless motors, switched reluctance motors and induction motors.
Electric motors are used in a diverse range of applications and one particularly challenging use is in electric power assisted steering systems. A motor, such as a three phase permanent magnet synchronous electric motor, is connected to a part of the steering system, typically the steering shaft that connects the steering wheel of the vehicle to the road wheels. A sensor, such as a torque sensor, produces a signal indicative of the torque applied to the steering wheel by the driver, and this signal is fed into a microprocessor. The microprocessor uses this signal to produce control signals for the motor which are indicative of the torque or current that is required from the motor. These control signals are converted into voltage waveforms for each phase of the motor within the microprocessor, and these in turn are transmitted from the microprocessor to a motor bridge driver.
The motor bridge driver converts the control signals, which are typically low level voltage waveforms, into higher level voltage drive signals that are applied to the respective phases of a motor bridge, usually separate from the bridge driver.
A typical bridge comprises a set of switches that selectively apply voltage from a supply to the phases of the motor as a function of the high level voltage drive signals applied to the switches from the bridge driver circuit. By controlling the switches the current in the motor can be controlled relative to the motor rotor position, allowing the torque produced by the motor to be controlled. The motor in use is thereby caused to apply an assistance torque to the steering system that helps, or assists, the driver in turning of the steering wheel. Because this torque effects the output of the torque sensor, this forms a type of closed loop control allowing accurate control of the motor torque to be achieved.
To help with the control of the motor, the microprocessor for PWM controlled electric motors, especially Permanent magnet synchronous electric motors, generally will also receive a measure of the current flowing through the windings or phases of the motor and this can either be done by means of separate current sensors for each of the phases, or by means of a single current sensor that is placed in the circuit so as to measure the total instantaneous current flowing between a D.C. power supply and the bridge circuit and motor combination.
The measured currents are typically converted within the microprocessor into a rotating d-q frame which rotates with the rotor, and then combined with the current demand signal, which is a function of the demanded torque and the characteristics of the motor, also in the d-q frame, indicative of the current that is demanded from the motor, to produce a current error signal. The error signal represents the difference between the current that is demanded in order to achieve a desired torque and the actual current flowing in the motor. The error signal is fed to a current controller which produces a set a voltage demand signal, also typically in the d-q frame, representative of the voltage to be applied to each phase of the motor that will best drive the error signal towards zero. The d-q voltage signal is then converted into PWM signals for the motor phases depending on which PWM strategy is used. The controller therefore acts to vary the PWM phase voltages in order to try to constantly minimise the magnitude of the error signal thereby ensuring that the motor current is as close as possible to the demanded current.
Motor drive circuits using feedback control and PWM are well known in the art. For example WO2006005927, discloses a typical system and the teaching of that document is incorporated herein by reference. The general layout of the control system is shown in FIG. 2 of the drawings.
It has been appreciated that with the system as described above there is a potential weakness in that a single component may fail leading to a loss of assistance from the motor. For instance, if the supply voltage to the microprocessor drops to a level at which the performance of the microprocessor is impaired, incorrect control signals may be provided. An internal fault in the microprocessor may also result in an incorrect function of the controller and lead to the microprocessor supplying incorrect command signals to the bridge driver circuit.
An object of the present invention is to ameliorate the possible problems associated with faults in the prior art arrangement described above.